The escape-wheel revolves in a clockwise direction. The two pallets _a_ and _b_ are so designed that only one can rest on the teeth at one time. In the sketch the sloping end of _b_ has just been forced upwards by the pressure of a tooth. This swings the pallet and the pendulum. The momentum of the latter causes _a_ to descend, and at the instant when _b_ clears its tooth _a_ catches and holds another. The left-hand side of _a_, called the _locking-face_, is part of a circle, so that the escape-wheel is held motionless as long as it touches _a_: hence the term, "dead beat"--that is, brought to a dead stop. As the pendulum swings back, to the left, under the influence of gravity, _a_ is raised and frees the tooth. The wheel jerks round, and another tooth is caught by the locking-face of _b_. Again the pendulum swings to the right, and the sloping end of _b_ is pushed up once more, giving the pendulum fresh impetus. This process repeats itself as long as the driving power lasts--for weeks, months, or years, as the case may be, and the mechanism continues to be in good working order. COMPENSATING PENDULUMS. Metal expands when heated; therefore a steel pendulum which is of the exact length to govern a clock correctly at a temperature of 60 deg. would become too long at 80 deg., and slow the clock, and too short at 40 deg., and cause it to gain. In common clocks the pendulum rod is often made of wood, which maintains an almost constant length at all ordinary temperatures. But for very accurate clocks something more efficient is required. Graham, the partner of Thomas Tompion, took advantage of the fact that different kinds of metal have different ratios of expansion to produce a _self-compensating_ pendulum on the principle illustrated by Fig. 203. He used steel for the rod, and formed the _bob_, or weighted end, of a glass jar containing mercury held in a stirrup; the mercury being of such a height that, as the pendulum rod lengthened with a rise of temperature, the mercury expanded _upwards_ sufficiently to keep the distance between the point of suspension and the centre of gravity of the bob always the same. With a fall of temperature the rod shortened, while the mercury sank in the jar. This device has not been improved upon, and is still used in observatories and other places where timekeepers of extreme precision are required. The milled nut S in Fig. 203 is fitted at the end of the pendulum rod to permit the exact adjustment of the pendulum's length. For watches, chronometers, and small clocks THE SPRING BALANCE takes the place of the pendulum. We still have an escape-wheel with teeth of a suitable shape to give impulses to the controlling agent. There are two forms of spring escapement, but as both employ a hairspring and balance-wheel we will glance at these before going further. [Illustration: FIG. 203.]